1. Field of the Invention
The present invention relates to conductive fine particles with a satisfactory monodisperse property, low cost, resistance to migration and excellent conductivity, as well as to a method for producing the conductive fine particles and an anisotropic conductive material using the conductive fine particles.
2. Related Background Art
Conductive fine particles are widely used as anisotropic conductive materials for, anisotropic conductive pastes, anisotropic conductive inks, anisotropic conductive adhesives, anisotropic conductive films, anisotropic conductive sheets and the like, by mixing and kneading with binder resins or adhesives.
Such anisotropic conductive materials are used in between opposing boards or electrode terminals for electrical connection between boards or for electrical connection of small parts such as semiconductor elements to boards in electronic devices such as liquid crystal displays, personal computers and cellular phones, for example.
Conductive fine particles disclosed in the prior art include conductive fine particles wherein a metal-plated coating layer is formed as a conductive film on the surfaces of non-conductive fine particles such as resin fine particles, which have uniform particle sizes and suitable strength, (see Japanese Unexamined Patent Application Publication SHO No. 63-190204, for example).
The conductive fine particles disclosed in Japanese Unexamined Patent Application Publication SHO No. 63-190204 have a nickel-plated coating film formed as a conductive film, but the phosphorus concentration is low during formation of the nickel-plated coating film. With such low-phosphorus concentration nickel-plated coating films (nickel-plated coating layer), a nickel-plated coating film with a crystal structure is formed.
With high-phosphorus concentration nickel-plated coating films, on the other hand, the conductivity is poorer than low-phosphorus concentration nickel-plated coating films, and therefore most of the known nickel-plated coating films are formed with an amorphous structures.
With the rapid advancement of high definition in liquid crystal displays in recent years, the bumps serving as the liquid crystal driving IC circuit electrodes are being designed with increasingly narrow pitches and narrow areas. Since the conductive fine particles must therefore be reduced in size and fewer particles can be captured on the electrodes, it has been necessary to lower the resistance of each conductive fine particle. In Japanese Unexamined Patent Application Publication No. 2006-028438 and Japanese Unexamined Patent Application Publication No. 2005-036265 there are disclosed conductive fine particles with improved conductive performance, having a gold layer as a precious metal formed on the surface of nickel.
Also, Japanese Unexamined Patent Application Publication HEI No. 10-101962 discloses conductive fine particles having shapes protruding through a conductive layer, wherein the protruding sections pierce the oxidized electrode surface to ensure conduction performance and remove the resin between the conductive fine particles and electrode, for an effect of lowering the connection resistance.
With narrowing pitches between circuit electrodes and narrowing areas, short failures occur more often due to bleeding and filling of anisotropic conductive adhesive conductive fine particles between adjacent circuits. Modifications have been proposed to maintain the insulating property between adjacent circuits, in order to avoid these problems.
In Japanese Patent No. 2748705 and International Patent Publication No. WO03/02955 there are described methods wherein macromolecular polymer core particles, that are coated with a gold layer, are further covered with insulating “child particles”. Also, International Patent Publication No. WO03/02955 discloses a method wherein the surface of a gold layer coating core particles is treated with a compound having a mercapto, sulfide or disulfide group to form functional groups on the gold layer surface. This allows rigid functional groups to be formed on the gold layer.